We know D.C. Get our free newsletter to stay in the know.

Just downstream from Chain Bridge, the corner of a large metal tank breaks the surface of the Potomac’s fuscous water. When the river runs high and hard, as it does today, the remainder of the iron or steel structure is barely visible. But when the river ebbs, the container emerges from a muddy eddy at the base of a rock-filled gorge. Oblong, the vat is studded with rivets. Small latched portals—like the doors on a baker’s oven—open into the watery cistern. The 30-foot-long tank is bisected at one end by a smaller cylinder, making the whole contraption resemble a sewage-pipe joint or a septic tank unearthed by erosion and swept down-river.

But this subaqueous container is neither a sewage pipe nor a septic tank. Nor is it an isolated bit of scrap metal. Several such receptacles are scattered down the Virginia riverbank between Chain and Key Bridges. Near each are clues to their past purpose—boulders pocked by inch-deep circular indentations and pierced here and there by small iron rings.

And right now, all around you—underfoot and overhead—are surfaces and facades linked to the Potomac’s submersibles.

A Washington City Paper T-shirt will be awarded to the person who writes the best humorous or factual explanation for the beached boiler. Inspired answers will appear in next week’s edition if they reach us by Tuesday morning. Submit your description, or suggest topics for this column, by writing to: Mysteries, Washington City Paper, 2390 Champlain St. NW, Washington, DC 20009. Our fax number is (202) 462- 8323, or e-mail us at Mysterieswashcp.com. No phone calls, please.